1. Field of the Invention
This invention relates to new photoresist compositions that can exhibit significantly improved shelf life and lithographic performance.
2. Background Art
Photoresists are photosensitive films for transfer of images to a substrate. They form negative or positive images. After coating a photoresist on a substrate, the coating is exposed through a patterned photomask to a source of activating energy such as ultraviolet light to form a latent image in the photoresist coating. The photomask has areas opaque and transparent to activating radiation that define a desired image to be transferred to the underlying substrate. A relief image is provided by development of the latent image pattern in the resist coating. The use of photoresists is generally described, for example, by Deforest, Photoresist Materials and Processes, McGraw Hill Book Company, New York (1975), and by Moreau, Semiconductor Lithography, Principals, Practices and Materials, Plenum Press, New York (1988).
More recently, certain “chemically amplified” photoresist compositions have been reported. Such photoresists may be negative-acting or positive-acting and rely on multiple crosslinking events (in the case of a negative-acting resist) or deprotection reactions (in the case of a positive-acting resist) per unit of photogenerated acid. In other words, the photogenerated acid acts catalytically. In the case of positive chemically amplified resists, certain cationic photoinitiators have been used to induce cleavage of certain “blocking” groups pendant from a photoresist binder, or cleavage of certain groups that comprise a photoresist binder backbone. See, for example, U.S. Pat. Nos. 5,075,199; 4,968,851; 4,883,740; 4,810,613; and 4,491,628, and Canadian Patent Application 2,001,384. Upon selective cleavage of the blocking group through exposure of a coating layer of such a resist, a polar functional group is provided, e.g., carboxyl, phenol or imide, which results in different solubility characteristics in exposed and unexposed areas of the resist coating layer.
With the desire to produce high-density semiconductor devices, there is a movement in the industry to shorten the wavelength of exposure sources and use deep U.V. radiation. Such photoresists offer the potential of forming images of smaller features than may be possible at longer wavelength exposure. As is recognized by those in the art, “deep UV radiation” refers to exposure radiation having a wavelength in the range of 350 nm or less, more typically in the range of 300 nm or less such as radiation provided by a KrF excimer laser light (248 nm) or an ArF excimer laser light (193 nm).
An important property of a photoresist is image resolution. A developed photoresist image of fine line definition, including lines of sub-micron and sub-half micron dimensions and having vertical or essentially vertical sidewalls is highly desirable to permit accurate transfer of circuit patterns to an underlying substrate. However, many current photoresists are not capable of providing highly resolved fine line images.
Another important property of a photoresist is photospeed, which can be defined as the exposure time coupled with the exposure energy required to activate the photoactive component, e.g. to generate a sufficient amount of photoacid to provide the desired solubility differential between exposed and unexposed areas of a photoresist coating layer.
It can be critical that a resist's photospeed is within an acceptable and consistent range or value to permit desired processing of the resist. For instance, sufficiently high photospeed is important in many processes, e.g. where a number of exposures are needed such as in generating multiple patterns by a step and repeat process, or where activating radiation of reduced intensity is employed. Sufficiently high photospeed also permits reduction in the concentration of the radiation sensitive component in the photoresist. On the other hand, a resist that is “too fast”, i.e. has too high photospeed, also can be undesirable. For example, an extremely high photospeed may compromise resolution of the patterned resist image, or exposure equipment may not be well suited to image such a fast resist.
Additionally, a consistent resist photospeed can be critical, e.g. so that a device manufacturer can use the same imaging conditions and obtain consistent results despite lot-to-lot differences of a resist product (such as precise amount and/or nature of the photoacid generator, polymer, etc.) that may frequently occur, particularly in large scale resist manufacturing processes. However, many current resists do not exhibit such consistent photospeed, and consequently a device manufacturer may either realize inconsistent results as different lots of a resist formulation are used, or the device manufacturer may be forced to carefully test the photospeed of each new lot of resist and then adjust the parameters of the exposure equipment to provide for consistent processing. Clearly, either alternative is undesirable.
Photospeed variations during storage also are indicative of resist degradation. For example, decreased photospeed of a resist upon storage can indicate degradation of the photoactive compound or other resist components. Storage stability is typically of high importance for a photoresist. Generally, after photoresist manufacture, a resist is stored for several months or more prior to use by a device manufacture. Any resist degradation during storage typically will only compromise lithographic properties.
It thus would be desirable to have new photoresist compositions that could provide highly resolved fine line images. It would be further desirable to have such new photoresist compositions exhibit consistent photospeed. It would be particularly desirable to have such new photoresists that show good storage stability, e.g. as indicated by consistent photospeed over time.